the Boss OC-2 is nice. good tracking, good range of tracking, and you blend in your main signal.
There's the MXR Blue Box - AWESOME octave down. Damn mean and gnarly. I like it a lot... but you can't mix your main signal in. Greater range of tracking than the OC-2 above. Plus, you can mix out the octave and have an insane wall-of-sound synth distortion.
Finally, the Foxrox Octron. Get this, don't have to do your octave up liao lah. Got 1 oct down - which is really all you need for gtr and 1 oct up - easily the best and most insanely rich/strong sounding oct up I've heard yet - and you can also mix in your main signal. What more do you want? Oh, true bypass... built like a tank... go listen to the clips on foxrox's website - the tracking is... as good as any other and the range is the same, if not more than, the Blue Box.